Knowledge of the lubricant film thickness between moving machine elements (bearings) is essential for bearing design in order to avoid premature failure, to achieve greater efficiency and to prolong the life of machines. Online measurement of lubricant films in bearings is desirable since this gives immediate feedback on machine performance. Lubricant films found in, engineering components have thicknesses ranging from hundreds of microns down to several nanometers. Measurements on these scales (especially at the thin end) are very difficult and have been a major research issue for many years.
There are a number of known techniques for lubricant film measurement and these have recently been reviewed [in Spikes H A. Thin films in elastohydrodynamic lubrication: the contribution of experiment. Proceedings of the Institution of Mechanical Engineers, 213 J:PP. 335–352, (1999)]. They can be divided broadly into electrical and electromagnetic methods.